Melamine production improvement



y 1966 ATSUO MURATA ET AL 3,251,843

MELAMINE PRODUCTION IMPROVEMENT Filed March 2, 1964 INVENTORS flLsuo Mar-Ha :70 Kcfabq 4 unea a sun 0 tornays United States Patent 3,251,843 MELAMINE PRODUCTION IMPROVEMENT Atsuo Murata, Ryo Kokubo, and Muneaki Matsui, all of Nei-gun, Toyarna-ken, Japan, assignors to Nissan Kagaku Kogyo Kabushiki Kaisha, Tokyo, Japan, a corporation of Japan 7 Filed Mar. 2, 1964, Ser. No. 348,341 2 Claims. (Cl. 260249.7)

This invention relates to an improved process. for continuous synthesis of melamine with a high yield from urea at high temperature and under high pressure. More particularly this invention relates to the prevention of corrosion of such reactors.

It is well known that melamine can be synthesized by heating urea under pressure, and such reaction must be carried out at a pressure of more than 60 atm. and a temperature of higher than 350 C., in order to attain a good yield of the desired product.

With such high temperature and pressure, reaction product tended to extremely corrode the material of the reactor, particularly at the beginning of the reaction. For this reason, it was essential, in industrial manufacture of melamine from urea, to select materials for the reactor having good corrosion resistance, and numerous proposals have hitherto been made.

For example, a platinum-ruthenium alloy or platinumiridium alloy has been proposed in the US Patent No. 2,542,762, and a reactor lined with titanium has been proposed in the U.S. Patent No. 2,644,820. Other reactors lined with silver and zirconium, etc., were also proposed. Although these linings have suflicient corrosion resistance during reaction of urea into melamine, these have the following serious drawbacks. V

The linings so far proposed can hardly be used because they have not suflicient strength at elevated temperatures due to difference in the thermal expansion coefiicient between the linings and iron and 'steel'materials constituting the main body of reactors. Practically, those materials cannot be used as heating pipe forsupply of heat desired for synthesis of melamine which is an endothermic reaction.

The present invention provides a process whereby the above difficulties are overcome and melamine can be manufactured effectively. The present inventors have found that, by using austenite stainless steel and hastelloy B as the materials for the reactor and bysupplying a small amount of oxygen together with urea into the reactor, corrosion of the reactor can be minimized under such severe conditions as a reaction pressure of over 100 atm. and a reaction temperature of over 400 C. The austenite stainless steel to be used for that purpose includes those consisting chiefly of 188 stainless steel elements and little amounts of special nonferrous metallic materials such as molybdenum and titanium, and all kinds of the austenite stainless steel having high chrome-nickel content. They retain sufiicient strength at elevated temperatures, and hence are suitable materials for reactors for use at high temperature and pressure.

Hastelloy B is an alloy composed of 26-30% of molybdenum, 47% of iron, and the rest of nickel. It is superior to austenite stainless steel in high-temperature strength.

The corrosion rates of these materials depend on the type of material and the reaction conditions to be employed, but in any case the rates are remarkably retarded by the addition of a small amount of oxygen to the feed.

Now, the result of a corrosion test conducted with a continuous reactor having a capacity of liters with test pieces inserted into the portions of the reactor where corrosion occurs most rapidly, is tabled hereunder by way of example. The reaction conditions employed were a pressure of atm. and a temperature of 400 C.

Degree of corrosion, in mm. of plate thickness With 1.3% With 0.3% With 0.2% Reactors material of 02 mixed of 0 mixed of Q2 mixed Without in per in per in per oxygen amount of amount of amount of C0 CO; CO 1 generated generated generated l 0-.. 4.0 3. 0 0. 65 0.76 Carpenter 20..-- 3. 0 1. 1 0. 28 0. 22 Hastelloy B 0. 31 o. 16

As will be apparent from the above table, introduction of a very small amount of oxygen can remarkably reduce corrosion. This is particularly true with austenite-type stainless steel. By suitably selecting the amount of oxygen to be mixed in, the degree of corrosion can be decreased to about a fraction of that obtained without the addition of oxygen.

The real reason for the remarkable decrease of corrosion as a result of introduction of oxygen is not thoroughly known. However, it is considered likely that the action of reducing gas produced by decomposition of traces of ammonia is lessened at a temperature of at least above 350 C. t

Generally speaking, the amount of oxygen to be mixed for prevention of corrosion is preferably equivalent to between 0.15 and 1.5% of CO gas generated during the reaction in which urea is .pyrolyzed to melamine, according to the following equation:

6CO(NH fiNHa 300;

If the amount of oxygen is less than 0.15%, the anticorrosive effect obtained is negligible, and if more than 1.5%, it is not only wasteful but may rather decrease the anti-corrosive effect. The amount of oxygen to be mixed, calculated against urea, is about 0.37 kg. by weight or about 200 liter by volume per kg. of urea at 20 C. at atmospheric pressure. Thus, the amount to be admixed may be either 0.3 to 3 liter of pure oxygen or 1.5 to 1.6 liter of air per kg. of urea.

It is well known that the presence of an excessive amount of ammonia in the pyrolysis of ammonia accelerates the production velocity and increases the yield of melamine.

If a little amount of oxygen is mixed, the presence of excess ammonia will have no effect on corrosion of the materials of the reactor.

Recently, a process for preventing corrosion of reactor materials which are subjected to high temperature and high pressure in a process of synthesizing urea from ammonia and carbonic acid gas has been proposed by the Japanese patent publication No. 4,879/ 1960. The process, characterized by the use of chrome-nickel steel containing 16% of chrome and 8% of nickel and by the presence of a little amount of oxygen, appears to be the same as the process hereby proposed by the present inventors. However, it will be clear from the following description that, in effect, the two are fundamentary different from each other.

It is plain from the specification thereof that the process proposed by the Japanese patent publication No. 4,879/ 1960 is intended to lessen corrosion of chromea nickel steel due to the attack by hydrogen sulfide in an aqueous solution of urea at a temperature of at most about 200 C., by dint of a little amount of oxidizing substance such as oxygen.

On the contrary, in a process for synthesis of melamine from urea, the conditions to be employed differ considerably from those for synthesis of urea from ammonia and carbonic acid gas. Austenite stainless steel and Hastelloy B will be badly corroded in the course of the process ata very high temperature of over 350 C., in a dry atmosphere substantiallyfree from a moisture content, and even when materials used contain no sulfuric compound as hydrogen sulfide which exerts the corrosive eifect. It isan amazing fact that, under such severe'reactive conditions, the corrosion can be lessened by the addition of a little amount of oxygen. The phenomenon has for the first time been found and adopted as an invention by the present inventors.

The present invention is illustrated by the following example.

Example This run is carried out in the reactor represented in the appended drawing. The reactor 1 has a capacity of 10 liters, a mm.-thick lining 2 of carpenter 20, an electric heating device 3, a pump 4 for introducing ammonia into the reactor, an ammonia heater 5, a pump 6 for feeding molten urea into the reactor, and an air compressor 7.

Molten urea was introduced into the reactor at a rate of 10 kg./hr. by means of the pressure pump up to 100 atm., together with ammonia in an amount equivalent to from 0.2 to 2.0% by weight of urea compressed also by the other pressure pump to 100 atrn. and heated to 400 C. by means of the heater 5. The reactor was heated by the electric heating device 3 until the temperature of the charge rose up to about 400 C., when melamine was obtained at a high yield of over 90+%.

When the reactor was operated continuously for 48 hours with no addition of oxygen, the average corrosion degree of carpenter determined on points where corrosion proceeded most badly was 3.6 mm. per year.

When,'under the same conditions as above, air was supplied at, a rate of 10 liters/hr. by means of the air compressor, or when oxygen in an amount equivalent to about one percent of CO generated was supplied to the reactor, the average corrosion degree in 48 hours of continuous operation was reduced to 0.5 mm./year.

Further, when the amount of air to be supplied was so adjusted that it was 2 liters per hour, or oxygen equal to amount of 0.2% of CO gas produced, the corrosion rate in 48-hour continuous operation was reduced to 0.3 mm./year.

What is claimed:

1. In a process for the continuous manufacture of melamine by successive pyrolysis of urea wherein the.

urea is heated at a temperature higher than 350 C. and under a pressure greater than atmospheres in a reactor made of a potentially corrosive material selected from thegroup consisting of austenite stainless steel and Hastelloy B, the improvement'comprising supplying an oxygencontaininggaseous medium to the reactor during the reaction. 7

2. A process according to claim 1 wherein the oxygencontaining gaseous medium is supplied to the reactor in a quantity sufiicient to maintain the amount of oxygen equivalent to between 0.15 and 1.5% of CO gas generated during the reaction.

References Cited by the Examiner OTHER REFERENCES Ofiicial Gazette of the US. Patent Ofiice, vol. 671,

pages 1500 and 1501 (1953).

WALTER A. MODANCE, Primary Examiner.

NICHOLAS RIZZO, Examiner.

M. w. WESTERN, JOHN M. FORD,

" Assistant Examiners. 

1. IN A PROCESS FOR THE CONTINUOUS MANUFACTURE OF MELAMINE BY SUCCESSIVE PYROLYSIS OF UREA WHEREIN THE UREA IS HEATED AT A TEMPERATURE HIGHER THAN 350*C. AND UNDER A PRESSURE GREATER THAN 60 ATMOSPHERES IN A REACTOR MADE OF A POTENTIALLY CORROSIVE MATERIAL SELECTED FROM THE GROUP CONSISTING OF AUSTENITIC STAINLESS STEEL AND HASTELLOY B, THE IMPROVEMENT COMPRISING SUPPLYING AN OXYGENCONTAINING GASEOUS MEDIUM TO THE REACTOR DURING THE REACTION. 